A spectrum analyzer is a device which displays a graph of relative power distribution as a function of frequency, typically on a cathode ray tube or chart recorder (see "Spectrum Analyzer Circuits," Measurement Concepts, .COPYRGT. TEKTRONIX, INC., 1969). This encompasses many different devices such as, for example, a properly programmed computer, a bank of filters, a swept or tuned filter, a superheterodyne signal-translation system, etc. Unlike an oscilloscope which may be capable of displaying the waveform of a single transient, a spectrum analyzer usually requires a repetitive signal in order to provide a satisfactory display. This is so because at any one instant, the receiver portion can only be tuned to a single component of a frequency spectrum. If a frequency present in a single transient is not in coincidence with the frequency to which the spectrum analyzer is tuned at the time of occurrence, it will not be displayed (see "Spectrum Analyzer Measurements," Measurement Concepts, .COPYRGT. TEKTRONIX, INC., 1969). To determine this frequency or to determine what frequency components may be generated along with the desired signal, several techniques have been devised such as down-counting the desired frequency by means of a single or doubled balanced mixers. This can be accomplished by heterodyning the signal to be measured with a second known stable signal, such as a crystal-controlled signal, to obtain the sum and difference of these two signals. The low frequency component, usually the stable signal, is removed from the output of the mixer by means of filters leaving only the sum and difference frequencies and their combinations. These signals are then mixed again with the original signal to be measured for recovery of the desired frequency.
The above briefly described process does have disadvantages, however. For example, the frequencies applied to the mixers must be attenuated in order to prevent complex spurious signals from being generated when they are applied to the second mixer and mixed with the original input signal. Additionally, precision filters must be used to remove the frequency components; the reason for filtering being noise consideration as is well known.